Re: How to efficiently handle DMA and cache on ARMv7 ? (was "Isget_user_pages() enough to prevent pages from being swapped out ?")

[Russell King - ARM Linux]

On Tue, Aug 25, 2009 at 08:53:29AM -0400, Steven Walter wrote:
> On Thu, Aug 6, 2009 at 6:25 PM, Russell King - ARM
> Linux<linux@xxxxxxxxxxxxxxxx> wrote:
> [...]
> > As far as userspace DMA coherency, the only way you could do it with
> > current kernel APIs is by using get_user_pages(), creating a scatterlist
> > from those, and then passing it to dma_map_sg().  While the device has
> > ownership of the SG, userspace must _not_ touch the buffer until after
> > DMA has completed.
> [...]
> 
> Would that work on a processor with VIVT caches?  It seems not.  In
> particular, dma_map_page uses page_address to get a virtual address to
> pass to map_single().  map_single() in turn uses this address to
> perform cache maintenance.  Since page_address() returns the kernel
> virtual address, I don't see how any cache-lines for the userspace
> virtual address would get invalidated (for the DMA_FROM_DEVICE case).

You are correct.

> If that's true, then what is the correct way to allow DMA to/from a
> userspace buffer with a VIVT cache?  If not true, what am I missing?

I don't think you read what I said (but I've also forgotten what I did
say).

To put it simply, the kernel does not support DMA direct from userspace
pages.  Solutions which have been proposed in the past only work with a
sub-set of conditions (such as the one above only works with VIPT
caches.)
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